The invention relates to a method for producing an infrared emitter from a quartz body having an endless form, wherein a reflector layer is deposited at least partially on the surface of the body made of quartz glass. The invention also relates to an infrared emitter produced in this way.
Components made of quartz glass are used for a plurality of applications, for example in lamp manufacturing for envelope tubes, bulb cover plates, or reflector carriers for lamps and emitters in the ultraviolet, infrared, and visible spectral ranges. Here, for generating special spectral properties, the quartz glass is doped with other substances.
Quartz glass is distinguished from other glasses by a low coefficient of expansion, by optical transparency across a wide range of wavelengths, and by high chemical and thermal stability.
When making lamps, the time constancy of power, the spatial orientation, and the efficiency of the output radiation play an important role. In order to minimize radiation losses or to direct the radiation selectively, optical emitters are provided with a reflector. Here, the reflector is either connected rigidly to the emitter or it may be a reflector component arranged separate from the emitter.
U.S. Pat. No. 2,980,820 describes a short-wavelength infrared emitter.
In German published patent application DE 198 22 829 A1, an infrared emitter is disclosed in which the lamp tube is constructed in the form of a so-called twin tube. Here, a quartz glass envelope tube is divided by a longitudinal crosspiece into two sub-spaces running parallel to each other, wherein a heating coil runs in one or in both sub-spaces. The side of the twin tube facing away from the main emission direction of the infrared radiation is coated with a gold layer, which acts as a reflector. This gold layer has, in the new state, a reflectivity of >95% across the entire infrared and withstands a continuous temperature of a maximum of 600° C. At higher temperatures, bonding losses and the evaporation of gold lead to a loss of the reflective property even after a short time.
In German published patent application DE 102 11 249 A1, a bright gold preparation is described that can be operated continuously up to a maximum temperature of 750° C. and, for a short time, far above this value, without resulting in the effects described above. Based on its composition, however, this gold features poor reflection of less than 70%, so that the effectiveness of this reflector does not satisfy the requirements placed on it.
Reflection layers made of gold with a high reflectivity of over 90% have, in general, the disadvantage that they are temperature stable only to a limited extent or else have a low reflectivity.
German published patent application DE 10 2004 051 846 A1 describes a quartz-glass component having a reflector layer. Here, the reflector layer is made at least partially of opaque quartz glass. To produce such a component with a reflector layer, it is necessary to apply the reflector to the empty emitter tube, in order to achieve the sintering of the layer as processing temperatures of 1250° and more are needed for the manufacturing process. At temperatures above 1100° C. quartz glass already softens noticeably. In particular, excess pressure in a quartz container then leads to inflation of the container. IR emitters are typically filled with argon at a pressure of 800 mbar to 1 bar, so that completed emitters would definitely be destroyed during the application of the reflector layer.
In the previously known method for producing emitters with a reflector layer, it is not possible to first coat the quartz body or the quartz tube and then to perform the pinching. The reflector can only be applied to the empty emitter tube, because the processing temperatures exceed 1250° C. Therefore, depending on the method, the reflector must be applied to the emitter tube before the beginning of the emitter production at the size required later. The reflector may not reach into the region of the pinched section. This is necessary, because the emitter tubes are heated uniformly with rotating burners during the pinching. In tubes with the described reflector layer due to the different amounts of quartz at the front and rear sides, either the coated side would not be adequately heated, in order to be able to pinch it, or the non-coated region of the tube would be heated too much, so that the quartz tube would become too viscous and would tear.
Typical pinching machines for filament bulbs are made of two opposing gas burners rotating about the quartz tube to be pinched. If the quartz tube is sufficiently hot for the pinching, then the two burners stop in their home position, so that the two pinching jaws can move together past the burners onto the quartz tube and in this way compress the quartz glass and seal molybdenum foil around it. The technique of pinching and using molybdenum foil is shown in German published patent application DE 29 47 230 A1.
Both burners are powered from a common supply line and thus essentially have the same burner output. The pinching can be triggered only when the entire tube has been sufficiently heated through. In this case, however, the part of the tube not covered with reflector material is becoming too viscous and starts to flow, so that the emitter can indeed usually be closed, but the shape of the pinched section is random and inadequate. In addition, very often non-sealed parts of the pinched section are observed, which are to be traced back to non-uniform temperatures of the glass or strongly deformed tube cross sections directly before the pinching. Using this method, the production output of emitters sufficient for sales could not be realized. Furthermore, the reject rate is very high, which increases the production costs.
If emitters with the same shape are to be produced in high numbers, then it can be tolerable with respect to production costs to individually coat already cut tube sections with the reflector material and to process them into emitters only after this point. The transition from the coated to the non-coated region then remains and indeed has a low-quality look and feel nearly independent of the application method, because it cannot be shaped economically to have a straight and clear construction—beads, spattering, cracks, threads, etc. negatively affect the visual impression.
In contrast, for a production of visually satisfactory emitters or for the production of small quantities of emitters with equal dimensions, the described method is complicated, very slow due to the finishing work that is often necessary, and expensive due to the plurality of tools and small batches.